Schwesig, D; Kalbitz, K; Matzner, E: Effects of aluminium on the mineralization of dissolved organic carbon derived from forest floors, European Journal of Soil Science, 54, 311-322 (2003)
Aluminium (Al) is abundant in soils, but the influence of Al on the mineralization of dissolved organic carbon and thus on carbon sequestration in soil is only poorly understood. We investigated the extent and rate of mineralization of dissolved organic carbon at various Al/C ratios. Dissolved organic carbon extracted from Oi and Oa layers under coniferous and deciduous forest was incubated with initial molar Al/C ratios from < 0.004 to 0.44 for 130 days. Mineralization was quantified by measurement of CO2. Rapidly and slowly mineralizable pools of dissolved organic C and their decomposition rate constants and half-lives (as a measure of labile and stable C) were modelled with a double exponential equation. Increasing initial Al/C ratios up to 0.1 led to a considerable decrease in mineralization (up to 50% compared with control samples). The half-life of the stable C pool increased up to 4-fold, whereas the half-life of the labile C pool was unaffected. Ratios of Al/C > 0.1 did not further decrease the mineralization, but led to increasing concentrations of free Al3+ in solution, and to increasing Al/C ratios in the precipitate, indicating that the Al complexation capacity of dissolved organic C was exceeded. Decrease in mineralization as well as formation of particulate organic matter (up to 56% of initial dissolved organic C) affected mainly the stable pool. Mineralization of dissolved organic C can be predicted from UV absorption by use of exponential regressions, but adding an Al variable did not improve the prediction significantly. We conclude that Al influences substantially the biodegradability of dissolved organic C percolating into the mineral soil, which may have consequences for the carbon sequestration in the soil. Declining Al concentrations would increase the mineralization of dissolved organic C only if the Al/C ratio becomes less than the 'threshold value' in the range of the Al complexation capacity of the dissolved organic C.
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